The present invention relates to fluid coupling devices, and more particularly, to such devices which are capable of changing between an engaged condition and a disengaged condition, in response to variations in a predetermined condition, such as engine compartment temperature.
A fluid coupling device of the type to which the present invention relates typically includes an input coupling member, and an output coupling assembly. The output assembly defines a reservoir chamber and an operating chamber, and the input member is rotatably disposed in the operating chamber. The forward and rearward surfaces of the input member, and the adjacent surfaces of the output assembly, cooperate to define a plurality of interdigitated lands and grooves, thus providing forward and rearward shear spaces.
In a typical fluid coupling device of the type to which the present invention relates, the output assembly includes a valving arrangement operable in response to variations in a condition, such as temperature, to permit fluid to flow from the reservoir chamber into the operating chamber, normally at a location disposed radially inward from the shear spaces. It is also typical to provide some form of pumping element disposed near the outer periphery of the operating chamber, operable to pump a small quantity of fluid from the operating chamber back into the reservoir chamber.
In prior art coupling devices of the type having both forward and rearward lands and grooves, filling of the forward and rearward shear spaces has normally been aided by a plurality of radially-extending grooves formed both on the opposite faces of the input member, and also on the adjacent surfaces of the output assembly.
One of the problems associated with fluid coupling devices of the type described above relates to the characteristic known as "response time". As used herein, the term "response time" refers to the ability of the coupling device to go from the fully disengaged condition to the fully engaged condition in a rapid and predictable manner. By way of example, when the coupling device is being used to drive the radiator cooling fan of a vehicle engine, it is important that, when the ambient air reaches the temperature at which the valving begins to open, the fluid entering the operating chamber is able to fill the shear space quickly enough. If the response time is too long, the engine may overheat, indicating to the vehicle operator that there is a malfunction in the cooling system, possibly necessitating shut down of the vehicle.
It should be noted here that the above-mentioned problem cannot be solved merely by lowering the temperature at which the fluid coupling device begins to engage. It would then be likely that, after initial engagement, the device would not subsequently disengage, thereby partially losing one of the primary benefits of using such devices for driving radiator cooling fans, i.e., the horsepower savings which occur when the device disengages.